In Chapter 4 we described the theory of a hypothetical ideal gas. The extent to which this endeavour has been worthwhile will become apparent as we compare the actual behaviour of real gases with the predictions of the theory. In the sections which follow we will familiarise ourselves with the experimental behaviour of real gases, and then develop ideal gas theory to compare its predictions with the experimental results. By juxtaposing predictions and experimental results we will see both the successes and the failures of ideal gas theory, and see how the theory can be developed to model the properties of real gases more closely.

§5.2 Density: As we saw in Example 4.1, ideal gas theory makes astonishingly accurate predictions.

§5.3 Heat capacity: If you had been studying physics at the start of the twentieth century rather than at the start of the twenty-first, you would have regarded the data on the heat capacity of gases as utterly mystifying. The explanation requires an understanding of the detailed structure and dynamics of the individual molecules that constitute the gas.

§5.4 Expansions: Gases can be compressed much more easily than liquids or solids which leads to interesting connections between their mechanical and thermal properties.

§5.5 Thermal conductivity: The key process in the transport of thermal energy through gases is collisions between gas molecules. In this section we will see that the ‘hard sphere’ model of molecular collisions we introduced in §4.3.4 needs to be significantly modified to explain the experimental data.

§5.6 Speed of sound: When we examine the data we will find that to explain the results we will again need to consider the detailed structure of the molecules of the gas. In particular we will find a connection with the heat capacity of gases that I still find surprising.

§5.7 Electrical properties: The insulating properties of gases are well known, but when subject to high electric fields gases break down and conduct electricity. At low electric fields, we will find that the properties of the gas again depend on the detailed distribution of electric charge within the molecules of the gas.

§5.8 Optical properties: The main optical property of gases is that they are almost completely transparent. It turns out that this property arises from an interesting relationship between the electronic properties of molecules and the process of evolutionary biology! When examining the data we will find interesting links to the electrical properties of gases, and to the detailed structure of the gas molecules.